1. Field of of the Invention
The present invention relates to an ac line filter which rejects high frequency common mode noise and normal mode noise over a wide frequency band, and is particularly useful for rejecting noise of about 40 MHz which is induced in commercial power lines from high frequency equipments such as a high frequency welder and comes into electronic equipments concerned, and for rejecting radiated low frequency noise of about 0.1 to several tens MHz which leak into the ac line from equipments such as a switching power supply.
2. Description of Prior Art
This type of conventional ac line filters have been of a type, for example, shown in FIG. 15 in which windings are provided on a closed circular magnetic path.
In FIG. 15 and FIG. 16 depicting the equivalent circuit of FIG. 15, the ac line filter has a pair of windings 4, 4' connected to terminals 1,1' and 2,2'. These Figures show that a load is connected to the terminals 1,1' and the ac line P is connected to the terminals 2,2', where the load L and ac line P may be interchanged. A core 3 as a closed circular magnetic path has effective permeability higher than a certain value across a frequency band, from a low frequency region to a desired high frequency region. The windings 4, 4' are wound in opposite direction to each other so that a common mode curent of a high frequency which flows into the terminals 1,1' or 2,2' causes a magnetic flux of the same direction. Capacitors 5,5' bypass a normal mode current of high frequencies in opposite phase flowing into the terminals 1,1' or 2,2'. Capacitors Cg bypass the high frequency common mode current flowing into the terminals 1,1' or 2,2' to the ground through a ground terminal 7. Capacitances Cs are stray capacitances between the input terminals and the output terminals of the respective windings 4,4'. Characteristics of rejecting the high frequency common mode noise current of a prior art ac line filter thus arranged is primarily governed by resonance characteristics of the bypass capacitors Cg and an inductance L+M (actually L+M is nearly equal to 2L) which is a sum of self inductances L of the respecive windings 4,4' and the mutual inductance M therebetween. However, when grounding effect of the ground terminal 7 is poor or the ground terminal 7 is not grounded, the aforementioned effect of the bypass capacitors Cg is lost and the noise rejection characteristics is solely dependent on the aforementioned inductances only.
The self inductances of the windings on the closed circular magnetic core described above is expressed by an equation L=.mu.SN.sup.2 /l where .mu. is permeability of the core, S the cross-sectional area of the core, N the turns of the windings and l the average length of the magnetic path. Thus if the inductance is to be made large to improve noise rejection characteristics, the number of turns N is usually increased because overall dimension of the filter is limited.
However, increasing the number of turns causes not only increased distributed capacitance of the windings but also stray capacitances Cs between the input and output terminals due to the fact that the closed magnetic path is a circular core, thereby deteriorating high frequency noise rejection characteristics.
Thus if the conventional ac line filter is used with the ground terminal not being grounded, then it will exhibit a poor noise rejection characteristics at high frequencies because the bypass capacitor is no longer effective. Further, even when the ground terminal is grounded, the filter still has the same problem that increasing the number of turns causes increasing stray capacitance Cs between the input and output terminals of the respective windings on a circular magnetic core, which stray capacitances Cg cause a poor high frequency characteristics.